linux-kernel - Re: [tip:locking/urgent] locking/lockdep: Disable cross-release features for now

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Date:   Tue, 17 Oct 2017 09:12:02 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     johan@...nel.org, arnd@...db.de, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        peterz@...radead.org, hpa@...or.com, tony@...mide.com,
        linux-tip-commits@...r.kernel.org, kernel-team@....com
Subject: Re: [tip:locking/urgent] locking/lockdep: Disable cross-release
 features for now


* Byungchul Park <byungchul.park@....com> wrote:

> On Sat, Oct 14, 2017 at 04:36:51AM -0700, tip-bot for Ingo Molnar wrote:
> > Commit-ID:  b483cf3bc249d7af706390efa63d6671e80d1c09
> > Gitweb:     https://git.kernel.org/tip/b483cf3bc249d7af706390efa63d6671e80d1c09
> > Author:     Ingo Molnar <mingo@...nel.org>
> > AuthorDate: Sat, 14 Oct 2017 09:26:59 +0200
> > Committer:  Ingo Molnar <mingo@...nel.org>
> > CommitDate: Sat, 14 Oct 2017 12:50:26 +0200
> > 
> > locking/lockdep: Disable cross-release features for now
> > 
> > Johan Hovold reported a big lockdep slowdown on his system, caused by lockdep:
> > 
> > > I had noticed that the BeagleBone Black boot time appeared to have
> > > increased significantly with 4.14 and yesterday I finally had time to
> > > investigate it.
> > >
> > > Boot time (from "Linux version" to login prompt) had in fact doubled
> > > since 4.13 where it took 17 seconds (with my current config) compared to
> > > the 35 seconds I now see with 4.14-rc4.
> > >
> > > I quick bisect pointed to lockdep and specifically the following commit:
> > >
> > >	28a903f63ec0 ("locking/lockdep: Handle non(or multi)-acquisition of a crosslock")
> > 
> > Because the final v4.14 release is close, disable the cross-release lockdep
> > features for now.
> > 
> > Bisected-by: Johan Hovold <johan@...nel.org>
> > Debugged-by: Johan Hovold <johan@...nel.org>
> > Reported-by: Johan Hovold <johan@...nel.org>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Byungchul Park <byungchul.park@....com>
> > Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Tony Lindgren <tony@...mide.com>
> > Cc: kernel-team@....com
> > Cc: linux-arm-kernel@...ts.infradead.org
> > Cc: linux-mm@...ck.org
> > Cc: linux-omap@...r.kernel.org
> > Link: http://lkml.kernel.org/r/20171014072659.f2yr6mhm5ha3eou7@gmail.com
> > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > ---
> >  lib/Kconfig.debug | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 2689b7c..e270584 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -1092,8 +1092,8 @@ config PROVE_LOCKING
> >  	select DEBUG_MUTEXES
> >  	select DEBUG_RT_MUTEXES if RT_MUTEXES
> >  	select DEBUG_LOCK_ALLOC
> > -	select LOCKDEP_CROSSRELEASE
> > -	select LOCKDEP_COMPLETIONS
> > +	select LOCKDEP_CROSSRELEASE if BROKEN
> > +	select LOCKDEP_COMPLETIONS if BROKEN
> 
> I agree with disabling crossrelease as default, becasue of regression,
> as I originally did.
> 
> However, it's expected to spend more time once it's enabled. Is the
> following acceptable?

No, please fix performance.

Thanks,

	Ingo